Seeds of a tepary bean cultivar (Phaseolus acutifolius A. Gray) contain two isoforms of alpha-amylase inhibitor, designated as alpha AI-Pa1 and alpha AI-Pa2, that differ in their specificity for insect alpha-amylases. Like alpha Al-1 and alpha AI-2 of common bean (Phaseolus vulgaris L.), the active form of alpha AI-Pa2 is a heterotetramer of two alpha and two beta subunits that are both generated from a common precursor, whereas alpha AI-Pa1 is composed of a single polypeptide. We have now isolated cDNAs encoding alpha AI-Pa1 and alpha AI-Pa2 and found that the deduced amino acid sequence alpha AI-Pa2 is most similar to that of alpha AI-2 whereas that of alpha AI-Pa1 most closely resembles those of seed lectins of tepary bean and common bean. Expression of these cDNA clones in azuki bean (Vigna angularis Willd. Ohwi & Ohashi) revealed that active forms of the inhibitor proteins accumulated in seeds and exhibited specificities for insect alpha-amylases identical to those of the native proteins purified from tepary bean seeds. The active form of alpha AI-Pa1 in transgenic azuki bean was a single polypeptide with a size similar to that expected for the full-length encoded protein. These results suggest that alpha AI-Pa1 defines a new type of bean alpha AI that is structurally related to lectins and is not activated by proteolytic processing. (c) 2005 Elsevier Ireland Ltd. All rights reserved.